Multiple cells are electrically connected in series and/or in parallel to produce a battery assembly having high output and high capacity. In order to accommodate ease of production, in general, a battery assembly utilizes battery modules where many cells are stored inside a case and used as a module within the battery assembly. In the battery module, multiple cells are connected electrically inside the case and negative and positive output terminals are provided to the outside from the case. Furthermore, based on the output and capacity required, a suitable number of battery modules are connected electrically in series and/or in parallel to produce a battery assembly. Therefore, in an assembly system where individual battery modules are assembled while fastening with screws, a complicated fastening operation is frequently required and may be inconvenient.